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Engine Specific Info and Questions => IDI Engine => Topic started by: TimpanogosSlim on April 23, 2014, 11:55:35 am
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not directly relevant to me because i am building with a brand new head, but a potential buyer of my old 1.5 head raised the question.
It turns out that the old 1.5 head should be milled down about 0.127mm to make it perfectly flat. That seems to be in the same neighborhood as the difference between thicknesses of head gaskets.
Assuming that the valve faces are flush or recessed, does a milled head require a thicker head gasket?
Also, the 1.5 head has marks that make it look as though it may have already been milled once. How would one go about verifying that?
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Milling does not require a different head gasket. Gasket thickness is based on piston protrusion.
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Milling does not require a different head gasket. Gasket thickness is based on piston protrusion.
That was my impression as well. granted that the valves are well seated and do not protrude from the face of the head while closed, it couldn't possibly be an issue.
My machinist was adamant that he could just take out the cam and run it through the mill assembled. Though he wondered if the steel pre-cups might require the use of the old stone mill rather than the fancy new one.
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Ja, them precups are inconel... an abrasive surfacing machine is required.
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Knock the precups out. Way less hassle, plus you don't want them any thinner than they already are.
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If they don't get surfaced, they gonna sit proud, no?
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They are supposed to.
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If you are taking off a lot, you can have the machinist take off some of the seat area in the head where the precups sit against. They should sit proud though as mentioned so don't take too much off.
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Remember you are talkng about 0.125m here or around 5 thou... Well don't, :o, as you only need to loose 2 thou to get to acceptable tolerance...
Don't knock your inconel's out, unless cracked, as you'll have to knock em back in again, and have the worry of securing them.
The composite face can be grinded, rather than milled.
The few thou can be taken out by hand by carborundum paper attached to several layers of plate glass, if you wish to do it yourself shimming under the glass with layers of news paper, to remove natural sag of a domestic table!
I used this method on my current engine, and only changed the head, because I forgot to tension the timing belt, and ran the engine for 7 miles with no tension on the belt at all, before belt jumped and snapped the cam. ::) Replacement cam came with free W German head, so I used that...
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I'm of the opinion milling ovals the hole and loosens them. Also tends to make some skip marks downstream.
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I'm of the opinion milling ovals the hole and loosens them. Also tends to make some skip marks downstream.
I'd tend to agree, that tool steel cutter would jolt as it left the ally and hit the steel.
As I'm no professional miller, is there as limit as to how fine a skim can be achieved, to minimise this?
Grinding seemed more practical for me...
I made over 50 passes of the head, in batches of 6 passes before lifting head and cleaning 'swarf', which reduced risk of inconel scrapings grooving the rest of the head...
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Not sure, but the gasket makers say not to go too smooth
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If they don't get surfaced, they gonna sit proud, no?
They are supposed to.
Gotta spec?
I just don't buy that... the gasket's crush ring crosses the precup.
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If they don't get surfaced, they gonna sit proud, no?
They are supposed to.
Gotta spec?
I just don't buy that... the gasket's crush ring crosses the precup.
Since VW says it's impossible to machine these heads, there are no specs, but if you look in manuals for other IDIs they sometimes have a spec. For some reason Isuzu comes to mind but I'm not sure if that's where I read it. When I had my head shaved the cups were sticking proud. It's still running great 5 years and 60k miles of pizza delivery driving later.
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Hmm interesting.
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